1. Field of the Invention
The invention relates generally to the field of analog to digital sigma-delta signal conversion.
2. Discussion of the Related Art
Discrete-time (DT) sigma-delta modulators have been successfully implemented using the switched-capacitor technique during the past decade. In a switched-capacitor implementation of a sigma-delta modulator, integrators are required to settle with an adequately small error at the end of each clock phase. This calls for fast amplifiers and thereby increased power dissipation. A second drawback of the switched-capacitor approach is aliasing of thermal noise and consequently increased in-band noise. Continuous-time (CT) sigma-delta modulation is an alternative way of converting analog signals to digital without the above-mentioned drawbacks. In CT-modulators, all integrators operate in the continuous-time domain and sampling occurs at the same time as quantization. As a consequence, CT-modulators are less demanding in terms of biasing current. Moreover, they provide an anti-aliasing filter without additional cost.
Despite their advantages, development of CT-modulators has been hindered by many practical issues, such as sensitivity to clock jitter, sensitivity to the shape of the feedback signal and inaccuracy of coefficients, all of which can result in inaccuracies in the outputs of CT-modulators.
Inaccuracy of coefficients stems from the fact that in a continuous-time structure such coefficients are set by two independent physical quantities such as resistance and capacitance. As a consequence, deviation of the coefficients from their nominal values can be as high as xc2x150%. Moreover, the value of the coefficients is prone to further variations due to temperature and aging. On the contrary, coefficients in a discrete-time system are set by the ratio of two devices of the same type, for example, capacitors. This shortcoming of continuous-time structures calls for a tuning scheme which should adjust some controllable variables in the system. A wide variety of tuning techniques for continuous-time filters can be found in the literature.
Inaccuracy of coefficients in a CT-modulator may cause several undesirable effects. The most obvious is departure of the loop function from its nominal characteristic and thereby degradation of noise shaping. The second problem is related to the dynamic range of the system and the maximum allowable swing of its internal nodes. This could result in harmonic distortion because of clipping and reduced dynamic range.
There is a need for the following embodiments. Of course, the invention is not limited to these embodiments.
In accordance with one aspect of the invention, a method for tuning a continuous-time modulator includes supplying a controllable current source, integrating a voltage from a voltage source using a discrete-time integrator to produce a discrete-time integrator output, continuous-time integrating the current from the controllable current source to produce a continuous-time integrator output, quantizing the difference between the continuous-time integrator output and the discrete-time integrator output to produce a quantizer output, controlling the polarity of the controllable current source with the quantizer output, counting the quantizer output to produce a feedback signal, and tuning the controllable current source as a function of the feedback signal.
In accordance with another aspect of the invention, an apparatus for a continuous-time modulator tuning circuit includes a switched-capacitor integrator in a fixed forward path, a continuous-time integrator in a feedback path, a quantizer coupled to receive input from the switched capacitor integrator and the continuous-time integrator, a counter coupled to receive input from the quantizer, a controllable current source coupled to receive input from the counter and to provide input to the continuous-time integrator, and an input voltage coupled to provide input to the switched-capacitor integrator.